Range of heartworm (Dirofilaria immitis) infection in Montana dogs

The serological presence of heartworm in dogs of Montana was studied in a 3-yr survey. Serum samples were provided by veterinary practitioners throughout Montana or were submitted to the state diagnostic laboratory. Sera from 3,490 dogs were tested using an enzyme-linked immunosorbent assay (ELISA) for circulating adult heartworm antigen. Twenty-four serum samples were positive for heartworm antigen. Two were from dogs that had never been outside the state. Nineteen additional positive dogs, 7 of which presumably had never been out of the state, were reported through correspondence with veterinary practitioners. Because suitable vectors for Dirofilaria immitis exist in Montana, there is potential for propagation of heartworm.